(257g) Adsorptive Separation In the Downstream of the Oxidative Methane Coupling Process

OCM is the reaction which produces ethylene by coupling two methyl radicals after the abstraction of one hydrogen atom from the methane molecule. Up to now, ethylene is produced by steam cracking higher hydrocarbons in valuable oil fractions such as naphtha. The long-term reserves exhaustion and short-term price fluctuation of crude oil are motivating ethylene producers to find alternative raw materials. Considering prices in Q4/2010 of methane (approximately 2.5 EUR/kmol) and ethylene (approximately 20 EUR/kmol), utilizing OCM to convert methane in natural gas (the world’s most abundant petrochemical resource) to ethylene (the world’s most produced organic compound) is economically attractive with fourfold added value. The challenges which render OCM industrially unrealizable are not only low conversion and selectivity of the reaction but also the high cost of the downstream separation. Therefore, an appropriate measure for separating components of the downstream, in particular ethylene – the desired product – represents a key requisite for advancing OCM to commercial level. This downstream mainly consists of ethylene, ethane, carbon monoxide, carbon dioxide, hydrogen and water, which are popular chemicals and theoretically easy to separate. In spite of that, the big capacities (due to low conversion of the reaction step) make the job a big challenge, where the effectiveness of the solution may determine if the whole process is profitable or not. Conventionally, each component in the product requires one separation step. Most proposed separation schemes separate components from the main stream (circulated methane) in the order of easiness and quantity: water, carbon dioxide, ethylene/ethane, carbon monoxide, and hydrogen. Since methane is the component in the highest quantity, these arrangements give all steps high duties, consequently high cost, in particular the ethylene separation step, which usually employs cryogenic distillation. Inspired by the success of adsorptive air separation in big scale (up to 250 tons/day) [1], this work looks into the possibility of replacing this energy-intensive distillation with adsorptive separation. Since this technique has been successfully applied at industrial level in case of separating CO₂, H₂O, and H₂, the idea can be pushed further by separating several components at the same time using an appropriate adsorptive separator. A plug flow model of fixed-bed adsorber was developed and several separation schemes were investigated via simulation. Among them, the simultaneously separation of ethylene and carbon dioxide using zeolite 4A is found realizable. Rough estimation showed that the technique requires a bed inventory which is more or less as that of commercial adsorptive air separation [1].

Acknowledgment

The authors acknowledge support from the Cluster of Excellency “Unifying Concepts in Catalysis” coordinated by the Berlin Institute of Technology and funded by the German Research Foundation – Deutsche Forschungsgemeinschaft.

[1]           R. T. Yang: Adsorbents: Fundamentals and Applications, Wiley, 2003.